Light emitting diodes (LEDs) have many advantages over incandescent, arc lamp and fluorescent light sources. Typically light emitting diodes produce less heat and in many applications can be more efficient than other types of light sources.
However, light emitting diodes have historically only been useful in low level lighting applications, e.g. indicator lamps, displays, and indoor signage.
The advent, however, of higher brightness LEDs have ushered in new applications where LEDs have been used in flashlights, traffic signal lights, outdoor signage, automobile headlights, and backlighting for flat panel displays. However, even with these higher brightness LED sources the luminance or brightness of the LEDs (lumens/unit area/unit solid angle) has not been sufficient to enable application of LEDs for high brightness sources used in projection displays and collimated light sources.
Current projection displays utilize high pressure arc lamp sources, which pose a safety hazard and also are short lived (500 to 5,000 hours). In contrast, LED light sources operate at low voltage, low pressure and are extremely long lived (up to 50,000 hours).
However, one problem with high brightness LEDs is dissipation of heat. Incandescent and arc lamp light sources dissipate most of their heat through infrared radiation. LEDs must dissipate their heat through thermal conduction. Therefore conventional LED light sources have utilized mounting configurations wherein adjacent LEDs cannot be too closely spaced in order to spread the heat dissipation over a larger area of the heat sink.
In fact, one of the primary limitations of creating even higher brightness LED light sources is the difficulty in dissipating the heat from the small area LED. LED brightness is directly related to the forward current through the light emitting PN junction. Higher currents produce higher lumen output and therefore higher brightness. However, higher current levels produce higher thermal loads and raise the temperature of the PN junction. Lumen output (brightness) is inversely proportional to the junction temperature.
Therefore, the present limit to driving an LED is determined by the ability to dissipate the heat generated by the LED. Therefore, a need exists to dissipate heat out of LEDs at higher rates than currently available. If a means of higher heat dissipation allows closer packing of the LEDs while driving them at higher current levels, a light source may be constructed that meets the brightness level requirements of projection displays, collimated light sources, and other applications currently being filled by high pressure arc lamp sources.
In a co-pending patent application, U.S. patent application Ser. No. 10/445,136, commonly assigned as the present application and herein incorporated by reference, a method is shown to achieve a high intensity light source by using LEDs in a light recycling cavity to achieve higher output luminance or radiance than could be achieved by a single LED. As described in U.S. patent application Ser. No. 10/445,136, the interior of the light recycling cavity is kept small such that the input light sources comprise a high percentage of the interior of the light recycling cavity. In practice, this requires a high packing density of LEDs. This, therefore, requires a means of dissipating the heat from these recycling light cavities.
Accordingly, it is the object of this invention to provide a means of dissipating heat from LEDs and configuring them in a highly dense packed array such as to provide a very high brightness light source that will rival that of high pressure arc lamps.
It is another objective of this invention that the LEDs are arranged in a configuration such that the output luminance of the light source is higher than the output luminance of any one of the LEDs making up the light source
It is another objective of this invention to create an LED based light source, which is a drop-in replacement for high pressure light sources used in projection displays. Example displays include digital-light-processor (DLP) displays, liquid-crystal displays (LCDs) and liquid-crystal-on-silicon (LCOS) displays.